Edit: Solution is further down the page. Though unless you are in for a challenge, you'd be better off getting a TI-82 from the later production runs (Look for TI-82 without golden-yellow lettering on the bezel).

On my TI-82 with a set of fresh batteries, I need to set the contrast to 9 so that the LCD produces a clear image, whereas a TI-83 produces a clear image at contrast 3 with the same set of batteries.
So far, I have performed a self-test on the calculator and removed all batteries including the backup coin for an hour, and it did not fix the display. I have yet to get around to checking ribbon cable resistance though.

Any clue to fixing this problem?
The self-test (assuming you do a display test) will only allow you to see what you already know, which is that the contrast is not to your liking Laughing

The early ti-82s didn't have the same display hardware as later ones. After rom 16.0, TI changed the display to increase the contrast. The hardware revisions A through K had the older, less contrasting displays. You can check the hardware revision by looking at the back of your calculator, the last letter is the hardware revision. If there is none, you either have an early "pre-A" revision, which had stickers instead of engravings, or you have a wonky parcus 82, which can also be pre-A.
However, it is not very likely that this is the case.

Checking the ribbon cable's resistance might be helpful in diagnosing the issue, but not very helpful for fixing it, as there is little that can reliably be done with regards to replacing it. There have been successful attempts, and countless unsuccessful ones. Every time I've tried it, I made things worse.

At contrast level 9, the screen should be entirely blue, and therefore unusable, so I find it very strange that yours would look normal at that contrast level. I happen to have an early ti-82 as well as some new ones, so I can confirm that this should be the case on both.

Honestly, when it comes to ti-82s, unless they are so broken that they aren't usable, I wouldn't suggest attempting a repair, especially in a case where like this where we aren't certain that the ribbon cable is at fault.
Thank you for the quick and detailed response.

My ti 82 has rom 16.0 and revision K (I-0194K).

The lower ribbon cable measures at 6.5 to 9 ohms towards the left and 8 to 13 ohms on the right. Unless I overlooked a quick fix, my TI 82 probably isn't worth repairing since they sell dirt cheap, around $10 for one with a perfectly working display on ebay.
The fat parts of the ribbon cable should measure just a few ohms. You could probably cut the two wide power traces and solder wires over them.
CVSoft wrote:
The fat parts of the ribbon cable should measure just a few ohms. You could probably cut the two wide power traces and solder wires over them.

Good idea. I'll try that when I find my soldering iron and I'm less busy.
update: the calculator display did not improve after soldering new wires in place of the power traces on the ribbon cable, unfortunately. The wires came from a stripped audio cable, repaired pieces measured 5 ohms.

edit: details
Having reached a conclusion, I would share it with those interested or googling. To my knowledge, this issue is inherent to the mainboard of specific hardware revisions, though I can only attest to revision 'K'.

1. Despite swapping the LCD with one from a revision 'Q' TI-82 (which did not exhibit this issue), the poor contrast persisted.
2. With fresh batteries, two other undamaged TI-82s (in a classroom/lab) of revision 'K' exhibited the exact same issue.
3. The need for fresh batteries in the TI-82 was documented by TI: https://web.archive.org/web/19990422205557/http://www.ti.com/calc/docs/faq/83faq088.htm

Therefore, the issue can only be fixed by getting an unaffected mainboard or replacing batteries frequently.
This is really interesting. CVSoft, can you explored/have you explored this reported issue, given your many TI-82s?
Pi Time wrote:
Therefore, the issue can only be fixed by getting an unaffected mainboard or replacing batteries frequently.

On a whim, I decided to replace the ROM 16.0 chip with a ROM 18.0 chip from a broken TI-82. And that fixed the contrast issue. I suppose something about the contrast-setting code changed after 16.0.

I've gotta say, those ROM chips are darn robust. This was my first time working with SMD chips and by the time I swapped the chip, I'd been sure it was toast from overheating. Nevertheless I continued and was proven wrong!
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